Many (almost all) systems need to be controlled.
When a system to be controlled comprises a plurality of machines, for example pumps, compressors, turbines, engines, two basic alternative approaches may be followed: centralized control or distributed control.
A centralized control system consists of a single controller controlling all of the machines; an example of centralized control system and method are disclosed in U.S. Pat. No. 3,133,502 where it is used for controlling a pump system comprising a plurality of pumps.
A distributed control consists of a plurality of controllers, typically one for each machine to be controlled.
U.S. Pat. No. 6,233,954 discloses a first distributed control system (considered as “prior art”) comprising a plurality of “local” controllers associated to a corresponding plurality of compressors and a “host” controller; the host controller gets measurement values from a single supply line pressure sensor and accordingly instructs the local controllers how to behave (i.e. the specific control to be carried out).
U.S. Pat. No. 6,233,954 discloses a second distributed control system (considered as “invention”) comprising a plurality of “local” controllers associated to a corresponding plurality of compressors; each local controller is connected to a discharge pressure sensor associated with the output of the respective compressor; the outputs of the compressors are connected to a single supply line; the control method comprises the steps of: designating one of the compressors as a highest ranked compressor, establishing a set point pressure threshold for loading and unloading each compressor, assigning a sequential ranking to said compressors beginning with the highest ranked compressor wherein the highest ranked compressor will initiate all commands for controlling all lower ranked compressors in the compression system, and repeating a loading subroutine until the discharge pressure of the highest ranked compressor is greater than the set point pressure threshold established therefor. Such a distributed control solution may be defined “hierarchic control” due to the ranking.
Another kind of distributed control solution is disclosed in patent application US 2003/0161731; a plurality of turbo engines cooperate in a station, and each turbo engine with the drive machine driving it forms a machine unit, with which a machine controller is associated; to control these turbo engines in parallel or tandem operation to observe at least one process variable, which is preset by the station and is common to all turbo engines, the preset, common process variable is set directly to each of the machine controllers, and this preset, common process variable is controlled exclusively via the machine controllers associated with the particular machine unit; there is no master controller; the total set point is sent, instead, to each of the machine controllers from a centralized set point presetter of the station directly via a signal line reaching all machine controllers; the actual value is likewise sent directly to each machine controller via a signal line, so that each machine controller can perform the necessary calculations on its own and can adjust the downstream control units just as if a common, higher-level master controller were used.
All the above described control systems and methods assume that each of the machines to be controlled is a single subsystem separate from each other; some of them, consider the possibility that there are one or more separate sensors used for controlling the machines. This is shown schematically in FIG. 1 wherein a set of N subsystems SPA-1 to SPA-N are controlled in a distributed way be a corresponding set of N controllers CPA-1 to CPA-N; each of the subsystems (SPA-1 to SPA-N) comprises a machine (MPA-1 to MPA-N) and a couple of auxiliary function assemblies (APA1-1 and APA2-1 for subsystem SPA-1 . . . APA1-N to APA2-N for subsystem SPA-N); for example the machines (MPA-1 to MPA-N) are pumps, the first auxiliary function assemblies (APA1-1 to APA1-N) are active lubrication equipments of the machines, the second auxiliary function assemblies (APA2-1 to APA2-N) are active sealing equipments of the machines; each of the controllers controls the machine and the auxiliary function assemblies of the corresponding subsystem.